JPH0693476B2 - Crystal defect evaluation apparatus and evaluation method by constant volume method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention is an apparatus for measuring a deep level formed by a defect in a semiconductor crystal by sweeping a sample temperature, and in particular, a temperature sweep rate is applied to the sample. The present invention relates to a crystal defect evaluation apparatus by a constant capacitance method, which is controlled by the time rate of change of a reverse bias voltage, and holds a depletion layer width, which is a measurement region, at a constant value.

[Conventional technology]

Conventionally, as a method of detecting a deep level formed by a defect in a crystal by changing the junction capacitance of a pn junction or a Schottky junction by changing the temperature of the crystal, TSC of Carballes et al.
There was the AP (Termally Stimulated Capacitance) method. T
The SCAP method continuously changes the sample temperature at a constant heating rate, and detects the junction capacitance change due to the charge state change of the defect of the depletion layer width due to the temperature change. Generally, the charge state of the energy level (hereinafter referred to as deep level) formed by a defect introduced into a crystal changes with an extremely long relaxation time, and therefore the nature of the defect, that is, It is not possible to determine the energy level, density, etc. of the deep level formed by the defect, and by varying the heating rate variously, when there are multiple deep levels, it is better than the shallow level density Nd. When the deep level density Nt is very small, the nature of the defect was determined under the condition that the TSCAP response can be obtained by each sum.

[Problems to be solved by the invention]

The TSCAP method changes the sample temperature at a constant temperature sweep rate and records the excessive junction capacitance change.Therefore, if the above assumption does not hold, it is converted to a defect energy level by the sweep rate. Since the sample temperature should change and the energy level cannot be accurately determined, and the change in the junction capacitance is observed under the condition of a constant bias voltage, the charge state of the defect changes and the junction capacitance changes. Changes, the depletion layer width, that is, the measurement area also changes,
If the defect density changes with the distance from the joint surface, there is a drawback that an accurate density determination cannot be performed.

[Means for solving problems]

FIG. 1 is a block diagram constituting the present invention. In the figure, a light source is a spectroscopic device for obtaining a predetermined unit light from the light source, and is a monochromatic light focused on a sample for irradiation. It is used for changing the deep-state charge state in a sample by optical transition and detecting the deep-level charge state changed by thermal transition. In order to apply a reverse bias by the operation of the bias voltage application circuit so that the difference between the junction capacitance of the sample measured by the capacitance meter and the capacitance value set in the comparison operation circuit becomes 0, a constant capacitance value is always set, That is, the width of the depletion layer is maintained and the measured region is always constant. In addition, the reverse bias voltage V _B (T ₁ ) applied to the sample at a certain sample temperature T ₁ is constant during the time set in the comparison operation circuit, as shown in FIG. If it is held at the value of, V _B (T ₁ ) becomes the final stable value V by the comparison operation circuit.
_It is determined that _B (T ₁ ) _s has been reached, and the sample temperature control device changes the temperature of the sample to the next measured temperature T ₂ .

[Action]

In order to overcome the first drawback of the prior art, the present invention does not make the temperature sweep rate constant, but keeps a constant temperature until a deep level charge state at a certain sample temperature T ₁ becomes a steady state, and thereafter, Change to the next sample temperature T ₂ . Therefore, unlike the prior art Is not constant, but changes with a deep level property, that is, a time constant determined by the electron and hole emission probabilities e _n and e _p .

Next, in order to overcome the second drawback of the prior art, in the present invention, unlike the prior art in which a change in junction capacitance with a constant reverse bias is detected with temperature, a junction for maintaining a constant junction capacitance is provided. The change in the reverse bias voltage applied to is detected. That is, taking as an example the case of increasing the temperature of a junction composed of an n-type semiconductor containing a deep donor level from a low temperature,
Since e _n >> e _p in the deep donor level, the deep donor level is filled with electrons and the charge state is neutral under a sufficiently low temperature condition of e _n 0. When the sample temperature rises to a temperature where e _n ≠ 0, excitation of electrons starts from the deep level to the conduction band, and the charge state changes from neutral to positive charge with the time constant 1 / τ = e _n + e _p. Transition to. In the prior art, since the bias is constant, when the deep rank changes from neutral to positive charge, the depletion layer width narrows and the measured region changes.However, in the present invention, the set capacitance value set in advance in the comparison operation circuit is set. Since the reverse bias voltage applied to the sample diode is applied so that, the depletion layer width is always constant and the measured region does not change.

〔Example〕

As a specific example, a measurement example of the recovery temperature of the photo-quenching level in the GaAs crystal will be described in comparison between the conventional TSCAP and the method according to the present invention.

In recent years, GaAs crystals have been widely used as materials for ultra-high-speed logic elements and optical communication elements because of their high electron mobility and optical characteristics. Unlike some silicon crystals, Ga and As
It is a compound semiconductor crystal consisting of two kinds of elements,
It is known that since the vapor pressure of As is particularly high, the ratio of Ga to As tends to deviate from 1: 1 and defects due to deviation from the so-called stoichiometric composition easily occur. In recent years, among the defects caused by the deviation from the stoichiometric composition, the deep level called EL-2 has attracted attention because it has a significant effect on the resistivity of crystals and the activation rate of ion-implanted impurities. ing. Therefore, clarifying the nature of this deep level called EL-2 and controlling its defects are important issues for both the GaAs substrate crystal providing side and the GaAs integrated circuit element manufacturing side. Has become.

About 1 eV is a characteristic of the deep level called EL-2.
Due to the irradiation of light in the vicinity, the ground state ▲ E ^° _T ▼ is changed to the excited state ▲ E ^* _T ▼, and the excited state ▲ E ^* _T ▼ is about 11
It has been reported that the temperature returns to the ground state (E ^° _T) at temperatures above 0K. In this example, the determination result of the transition temperature from the excited state to the ground state in the deep level called EL-2 will be described by the result of measurement by the conventional TSCAP method and the method according to the present invention.

The sample is an n-type none-doped HB GaAs crystal, and 50
An ohmic contact was obtained by vapor deposition of AuGeNi / Au with a diameter of 0 μm and sintering in H ₂ at 450 ° C. for 30 seconds, and a surface obtained by obtaining a Schottky junction with Al by electron beam vapor deposition was used. Of course, the structure of the sample is not limited to the above, and it is also applicable to the structure having a pn junction. Of course, the material and the structure of the sample are not limited to the GaAs crystal having the Schottky junction, but are widely used in semiconductor crystals. It can be widely applied as long as there is a junction formed in part of the diode or the transistor formed in 1. and a junction whose depletion layer width is controlled by the applied voltage.

First of all, the sample is measured from the excited state ▲ E ^* _T ▼ to the ground state ▲ E ^°
Temperature lower than the temperature at which recovery to _T ▼ occurs (eg 45K)
The sample is cooled to a dark place, a reverse bias voltage is applied at low temperature, and light of about 1 eV is irradiated, and the EL-2 level is changed from the ground state ▲ E ^° _T ▼ to the excited state ▲ E ^* _T ▼. Move. After that, while irradiating with 0.75 eV of light, the sample temperature is raised at a constant rate of temperature rise in the prior art and while detecting the time change rate of the bias voltage in the present invention. At a certain recovery temperature T, the excited state ▲ E ^* _T ▼ of EL-2 becomes the ground state ▲ E ^゜ _T ▼
Recover to. It is known that the ground state ▲ E ^° _T ▼ has a level near 0.75eV from the conduction band. Therefore, if it is irradiated with 0.75eV light, the junction capacitance will increase in the conventional technology when it is recovered. In the present invention, an increase in reverse bias voltage is detected.

In FIG. 3, (a) is an example in which the recovery temperature of the level called the excited state of EL-2 is measured by the above-mentioned method with a temperature sweep rate of 1.4 K / min by the conventional TSCAP method. In Fig. 3, (b) is a measurement example with a temperature sweep rate of 0.48K / min. In Fig. 3 (a), about 130K, and in (b) about 120K, the junction capacitance due to the recovery phenomenon of the photo-quenching level. Has been detected, the problem of the prior art that the detection of the temperature at which the charge state changes due to the temperature sweep speed greatly changes appears. It goes without saying that the width of the depletion layer, which is the measured region, changes with temperature because the junction capacitance changes.

On the other hand, FIG. 4 shows the results obtained by the present invention under the above-mentioned conditions. Fig. 4 (a) shows the result of the measurement under the above-mentioned conditions without irradiation with light of 1.1eV and only with light irradiation of 0.75eV, and Fig. 4 (b) shows the result of irradiation with light of 1.1eV. It is the result of measurement by light irradiation of 0.75 eV after photoquenching. The measurement result according to the present invention in FIG.
At 111K and 112K, a remarkable increase in the reverse bias voltage required to hold the capacitor at a constant capacity was observed, which clearly shows that there are at least three levels having recovery temperatures.
Moreover, it goes without saying that the width of the depletion layer, that is, the measured region is kept constant during the measurement of FIG.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY As described above, the present invention has the effect of being able to detect the temperature at which a change in the charge state of a deep level occurs extremely accurately while keeping the measured region always constant.

[Brief description of drawings]

FIG. 1 is an apparatus configuration diagram of the present invention, FIG. 2 is a schematic diagram of a sample temperature and bias voltage sequence showing a measuring method of the present invention,
FIG. 3 shows a measurement example according to the prior art, and FIG. 4 shows a measurement example according to the present invention.

Claims (2)

[Claims] 1. A light source, a spectroscope that obtains monochromatic light from the light source, an optical system that converges and illuminates the monochromatic light onto a sample, a temperature control device that controls the sample temperature to a constant value, and A capacitance meter for measuring capacitance, a comparison operation circuit for comparing the value of the capacitance meter with a set capacitance value, and a bias voltage application circuit for applying a reverse bias voltage to the sample according to the operation result of the comparison operation circuit, The temperature control device obtains a saturation value of the reverse bias voltage at a constant sample temperature, and then controls the sample temperature to any of a plurality of other preset constant temperatures. Crystal defect evaluation device by the method. 2. A junction capacitance of a sample at a constant temperature is made equal to a set capacitance value by applying a reverse bias voltage to the sample, a saturation value of the reverse bias voltage value is obtained, and the reverse bias voltage is A method for evaluating a crystal defect by a constant volume method having a process of controlling a sample temperature to a plurality of other constant sample temperatures after being saturated.